The present invention relates to a method and to apparatus for using a laser to machine light guides, to light guides obtained thereby, and to back-lit screens incorporating such light guides.
The invention applies more particularly to light guides in the form of a generally plane than plate constituted by a material that is translucent or transparent, in particular glass, polycarbonate, or polymethyl methacrylate.
Such light guides are used in particular for making luminous display devices such as signs, and in making display terminals for computers and similar electronic apparatuses, in particular for back lighting liquid crystal screens. In those applications in particular, the light produced by a source is injected into the light guide via one or more of its side faces (edges), with the light leaving the light guide via one (or both) of its two main faces that are generally parallel to each other.
In those applications in particular, it is desirable for the light flux density leaving the main face to be as uniform as possible. It is also desirable to avoid or at least limit losses or leakage of light. In order to back-light a liquid crystal (LCD) display, it is essential for such performance to be optimized and for mass, size, and cost to be minimized.
To manufacture such light guides, it is known for the main face of the light guide that is to radiate light and that is to be placed against the rear face of the LCD display to have formed therein, generally by molding, projections and/or depressions of regular shape, e.g. of prismatic shape, and which are regularly distributed over all or part of the surface of said main face. Another technique consists in forming xe2x80x9cspotxe2x80x9d masks on said face constituted by a covering applied to said face and pierced by orifices to allow light to pass through.
It is also known from document EF-A-0 945 674 to machine linear diffusing patterns at regularly variable spacing by means of a laser beam.
An object of the present invention is to provide an improved method of manufacturing such visible light guides.
Another object is to propose apparatus for implementing the method.
Another object is to propose improved light guides and display devices incorporating such light guides.
To this end, in a first aspect, a main face of the light guide that is to diffuse (visible) light is exposed to laser radiation that is sufficiently/intensive and/or sufficiently prolonged to form a deep (central) depression in the surface of said main face together with a plurality (several tens or hundreds) of irregular xe2x80x9cperipheralxe2x80x9d projections and/or depressions disposed on either side of said central depression (and/or surrounding it), of amplitude (in area and in relief) that is generally irregular and less than the amplitude of said central depression.
In another aspect, a main face of a light guide is exposed to laser radiation that is sufficient to form a plurality of surface projections (and respective depressions) that differ from one another in shape and height (or depth), and to cause non-uniform zones to be formed within the light guide and substantially in register with said projections and depressions, which zones differ from one another in size, shape, and refractive index.
To this end, said main face is preferably subjected to radiation of intensity lying in the range 104 watts per square centimeter (W/cm2) to 107 W/cm2, and in particular in the range 3xc3x97104 W/cm2 to 3xc3x97106 W/cm2. To obtain a concentrated laser beam from the beam emitted by a commercially available source, a concentrator is interposed on the path of the beam, said concentrator preferably being a parabolic mirror. Means are also provided for maintaining the mirror at a predetermined distance from said main face.
The presence of such deformations and irregularities both at the surface of the main face of the light guide plate and within it, gives rise to improved and regularly distributed diffusion of the light that leaves via said main face of the light guide. It seems likely that the presence of the irregular projections and depressions contributes significantly to the observed good performance as measured on samples processed in accordance with the invention.
To this end, it is important to obtain a beam at the outlet from the concentrator that includes a focal zone (or spot) of small dimensions. The diameter of the spot at the focus preferably lies in the range 10 microns (xcexcm) to 200 xcexcm, and in particular in the range 25 xcexcm to 100 xcexcm.
It is also important to maintain the focal plane containing said spot in the immediate vicinity of the surface to be machined, in particular at a distance therefrom that is of the order of xc2x110 xcexcm to xc2x1100 xcexcm.
To this end, and in particular when machining transparent plate materials of thickness that varies from one plate to another and/or varies from one area to another in a given plate, it is necessary to provide dynamic concentrator-positioning means for positioning the concentrator relative to said surface so that the distance between the surface of the plate and the focal plane remains within said limits. To this end, in a preferred embodiment, the mirror is mechanically linked to a part that forms a pneumatic bearing member that bears against said surface so as to obtain a constant distance between the bearing member and the surface (which distance generally lies in the range about 10 xcexcm to about 100 xcexcm).
In a preferred embodiment where the central depression extends along a continuous rectilinear line, it forms a gutter having projecting zones (or side strips) extending along each of its sides, which zones include depressions that are irregularly shaped and located. This gutter (or groove) is preferably 5 xcexcm to 50 xcexcm deep, 5 xcexcm to 200 xcexcm wide, and in particular about 50 xcexcm to 100 xcexcm wide, and each of said zones extends over a width that is substantially equal to the width of the central gutter. The mean width of a diffusing pattern (including said groove and the two side strips) is preferably greater than 100 xcexcm and less than 400 xcexcm, and in particular lies in the range 100 xcexcm to 250 xcexcm.
The machining apparatus preferably comprises:
a bench suitable for supporting a plate of transparent material to be machined;
a laser source suitable for emitting a laser beam and mounted in fixed relationship to the bench;
one or more mirrors for reflecting said beam and directing it substantially parallel to the bench; and
said beam concentrator which is mounted to move relative to the bench along at least two orthogonal directions (two axes X and Y), so as to be able to move in particular along a grid of parallel straight line segments so as to form parallel diffusing grooves;
said concentrator (or parabolic mirror) being also mounted to move (relative to the bench) along a third direction (their axis Z) that is generally orthogonal to the first two directions, thereby enabling its outlet focal plane to be brought into coincidence with the surface to be machined.
A beam expander, e.g. constituted by a pair of coaxial lenses having different focal lengths, is preferably disposed between the source and the concentrator, and is preferably mounted in a fixed position relative to the bench. The expander serves to reduce the size of the focal spot delivered by the concentrator by increasing the aperture of the beam.
A diffusing and refracting pattern is machined on the surface of and within the plate by moving the outlet focal spot from the concentrator over the surface of the plate. In particular, this machining is obtained by moving a parabolic mirror which is maintained by means of a cushion of air (or some other gas) at a constant distance from the surface to be machined, which distance corresponds to the focal length. The speed of this displacement is preferably selected to lie in the range 0.2 meters per second (m/s) to 4 m/s, so as to match the energy density of the beam to the xe2x80x9chardnessxe2x80x9d of the material that is to be machined, and also to the mean depth and width desired for the depressions that are formed.
The use of a parabolic mirror also makes it possible to improve focusing of the beam as reflected by the mirror so as to form a small focal spot in the focal plane.
In the preferred case where light injection takes place via a single longitudinal edge of a rectangular light guide plate, said plate is fitted with a mirror extending against (and facing) the longitudinal edge thereof which is opposite to the edge through which light is injected into the light guide. For back lighting an LCD display, it is preferable to use a row of LEDs as the light source, in particular of LEDs that emit xe2x80x9cwhitexe2x80x9d light or else a series of triplets of LEDs emitting light in three colors enabling additive combination to be performed (red, green, blue, red, green, blue, . . . ). In order to reduce the size of the display device, it is possible to use LEDs integrated in an elongate CMS circuit of a shape that corresponds to the shape of the edge, and it is possible to use a light guide plate of thickness lying in the range about 1 millimeter (mm) to 2 mm.
Alternatively, the light source can be constituted by a tube such as a cold cathode fluorescent tube (CCFT).
The main (emitting) face of the light guide can have a plurality of diffusing patterns in the form of points (or spots). Nevertheless, these patterns preferably extend in the form of straight line segments parallel to the edge through which light is injected, these segments being of substantially constant profile (width and height), and being spaced apart substantially regularly in non-monotonic manner: the distance between two adjacent segments decreases on going away from the light injection edge towards an intermediate zone of the light guide between said injection edge and the reflection edge extending along the opposite side of the light guide, after which said distance increases on going away from said intermediate zone towards said reflection edge. This decrease and increase preferably takes place substantially in the form of a geometric progression (e.g. to within 10%).
In a variant, the linear diffusing patterns can be spaced apart substantially regularly. Under such circumstances, it is the width and/or the depth of said central depression (or groove) that is caused to vary in non-monotonic manner from said injection edge towards said reflection edge, and to vary in inverse manner to that described above concerning the distance between grooves that are identical. It is possible from one diffusing pattern to another to vary the amount of energy that is radiated, for example by varying the power delivered by the laser source and/or by varying the spacing between the outlet focal plane from the concentrator and the surface to be machined.
These non-monotonic variations are also applicable when two sources are provided that inject light via two opposite edges of the light guide plate. When only one (linear) light source is used and when the opposite edge is not fitted with a mirror, it is possible to use spacing that varies monotonically, as described in document EP 0 945 674.
The excellent efficiency and very good light diffusion as delivered by the light guide can, in certain applications, make it possible to omit the diffusing screen that is usually provided for back lighting an LCD display and which is generally disposed between the light guide and the display or else in front of the display.
It has also been observed that light guides of the invention are particularly suitable for obtaining highly uniform diffusion of the light that comes from a plurality of sources (LEDs) in spite of the discontinuous (non-uniform) nature of the light flux injected through the edge in such a configuration.